Hybrid rocket propulsion, because of its safety and potential to create high performance rocket motors, offers a wonderful opportunity for the construction and test firing of both rocket motors and actual flight vehicles. Hybrid rockets use a liquid oxidizer, usually non-toxic nitrous oxide, and a solid fuel, such as rubber, to create a rocket engine. The advantage of this is that both fuel and oxidizer are safe until the oxidizer is injected into the engine.
Ethanol is a vital new renewable fuel for the future, and when derived from sugar cane can return 8:1 in energy invested in its recovery. Ethanol also has a long and illustrious record as a rocket fuel, being the fuel for the German V-2 missile developed by Werner Von Braun, as well as the Jupiter C that orbited the first United States satellite, and the American Redstone Rocket that carried the first two Americans into space, Alan Shepard and Gus Grissom. Thus, ethanol is an excellent rocket fuel and it is desirable for use in a hybrid rocket.
Hybrid rockets use a liquid oxidizer and solid fuel, so, in order to use ethanol in a hybrid rocket, it must be made into a solid material, such as the commercially available product known as STERNO®, a petroleum based solid fuel which uses calcium acetate as a gelling agent. The commercial STERNO® fuel product has been demonstrated to be a good hybrid fuel; however, commercial STERNO® gel does not have sufficient strength to hold its shape during the acceleration of the rocket burn.
Methyl Hydroxyl Propyl Cellulose (MHPC) has been made into a gel with ethanol as a fuel medium, as described in U.S. Patent Application Publication 2003/0217504. However, this fuel medium is limited to a “fuel for cooking and barbeque/fire lighter units.”, abstract, and has nothing to do with solid fuel for hybrid rockets.
Ethanol based liquid fuels are currently being used for internal combustion engines, as discussed in WO1998/056878 to Craig, et al. but this has nothing to do with solid fuel for hybrid rockets.
Liquid monopropellant fuels are also known and reported in WO2001/009063 and U.S. Pat. No. 6,984,273 to James D. Martin et al.
A cryogenic solid hybrid rocket engine and method of propelling a rocket is reported in U.S. Pat. No. 6,101,808 to Knuth et al. The solid fuel for a hybrid rocket is achieved by taking the fuel that is liquid or gaseous at room temperature and then freezing it into a solid for use in a hybrid rocket. This is a very expensive and energy intensive fuel preparation process.
None of the prior art references is entirely without technical merits. However, there remains a need for low cost, easily prepared and efficient solid rocket fuels for hybrid rocket engines. The present invention provides the needed solid fuel.